Judge Roy Moore is not "Congress" nor is he establishing a religion. If anything, it would be more accurate to say Judge Moore's rights are being violated. He is being prohibited from the free exercise of his religion.
Of course the argument can be made that Judge Moore is perfectly free to believe The Ten Commandments......for now.
I don't have a problem with the Ten Commandments monument, and I don't think it's unconstitutional, but I don't think your argument will fly...
If another judge wanted to put up a Wiccan or Buddhist or Hindu monument next to Judge Moore's monument (which is on public property, not the Judge's personal property), would you say the other Judge had a right to free exercise of his religion as well?
Not quite. It also prohibits Congress (the only law making body of the gov.) from making any laws respecting an establishment of religion meaning it can't make laws having to do with how the people establish religion thus the wording of it.
Amendment ICongress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
Notice how it says 'an establishment of religion' not 'the establishment of religion'. It covers both aspects completely. Congress can make no law instituting gov. mandated religion and no law respecting (concerning) religions that are established.
The legal reasoning is this. The First Amendment is binding on the states. Whether or not you agree is interesting but irrelevant. This has been repeatedly reaffirmed by the US Supreme Court, which is why you have 1st Amendment rights in every state. Normally, I would expect all of us to like that.
So now the STATES may not establish a religion.
Well, that would apply to state courts, would it not? They are created and funded by the states.
Now, the legal question comes down to what "establishing" a religion is. This is where the debate can continue. "Establishing" has been held by the courts as "official expression for one religion over another." That's not a quote from court case. It's a paraphrase.
What Judge Moore did with his monument, in the state-funded courthouse, was to pretty much blatantly promote his view about religion and law.
Regardless of how correct he might be, it's pretty much a blatant violation of the Constitution, or at least how it's been pretty much been construed over many decades.
That's why we're where we are at today.